Electrostatic powder coating is well known. In such painting, paint powder is sprayed onto an article to be painted. Due to the article and the paint particles having different electrical potential as by one or the other being charged, the paint powder particles adhere to the article, coating it. The article is then moved to, for example, an oven where by baking, the paint powder particles are fixed into a continuous paint coating.
The article is sprayed with the paint powder in a paint booth which permits over spray of paint powder to be collected and re-sprayed. To keep the paint powder in the paint booth and collect the paint powder, paint booths are kept under a partial vacuum as, by drawing air with a relatively large capacity fan from the booth through filters which collect the over-spray paint powder. Paint powder caught in the filter may by gravity fall from the filters into a paint powder collection hopper. To assist in unclogging of the filters and dislodge paint powder therefrom, it is known to periodically pass air under pressure backwards through the filters. Such backflow cleaning with pressurized air has a number of disadvantages. Firstly, to provide air under pressure requires a compressor which is expensive and bulky. The inventor has appreciated that the use of a compressor is especially disadvantageous where a number of modular paint booths are desired for easy substitution and either a compressor is required for each module or connections to a central compressors must be made to switch modules. Secondly, subjecting the filters to strong pressure differentials between the vacuum on one hand and the above atmosphere pressure on the other hand causes a loud noise or pop which is startling and disturbing to paint booth operators. Thirdly, it is believed that to have a strong pressure differential between a vacuum condition and a pressurized condition unduly stresses the filters and reduces their life.